To paraphrase certain TV ads, I needed cash now (had to buy a warrior). The game was on turn 36 and the only known other civ was Arabia, who was neutral towards me. I offered 1 gold/turn in exchange for 25 up front, and he countered with 22. Since that was still enough I took it. Later I pulled up a spreadsheet and ran the numbers. Using a simple internal rate of return calculation, Harun's loan/investment in me yielded 2.1%, which is not that great in real-world terms. Plus that presumes we don't go to war in the next 30 turns. A friendly AI might even accept a better deal (not sure). Regardless it seems like a worthwhile trade if your future cash flow can sustain it. Just a random thought, others may have already analyzed this far more already.